/Engine/Plugins/Runtime/CommonUI/Source/CommonUI/Public/CommonLoadGuard.h
Virtually identical to aUBorderSlot, but unfortunately that assumes (fairly) that its parent widget is aUBorder.
[UCLASS](Programming/UnrealArchitecture/Reference/Classes#classdeclaration)() class ULoadGuardSlot : public [UPanelSlot](API\Runtime\UMG\Components\UPanelSlot)